1. Field of the Invention
The present invention relates to a semiconductor material, and particularly to a semiconductor material for solar cells.
2. Description of the Prior Art
Photovoltaic devices can be generally classified into crystalline silicon solar cells and thin film solar cells. Crystalline silicon solar cells have mature manufacturing technology and high efficiency; however, the material and manufacturing cost high. Thin film solar cells are made by forming a light absorbing layer on a non-silicon substrate, such as glass substrate, and cost relatively low. Thin film solar cells can be further classified by material of the light absorbing layers, such as amorphous silicon, cadmium telluride (CdTe), copper indium gallium sulfide/selenide (CIS or CIGS), dye-sensitized film (DSSC) and other organic films.
The quaternary semiconductor Cu2ZnSn(S,Se)4 (also referred to as CZTS), having a multi-crystalline structure similar to CIGS, is new photovoltaic material which attracts interests recently due to its low cost, natural abundance and non-toxic elements. Conventional methods for forming CZTS films are carried out under vacuum environment. It is reported that Ito and Nakazawa prepared CZTS thin films on a stainless steel substrate by atom beam sputtering. Friedl Meier et al. prepared CZTS thin films by thermal evaporation. Katagiri et al. prepared CZTS thin films by RF sources co-sputtering followed by vapor phase sulfurization or by sulfurizing electron-beam-evaporated precursors. Vacuum processes are in general quite expensive and the cost of the CZTS solar cells is thus increased. U.S. Patent Application Publication No. 2012/0282730 and No. 2012/0280185, the entire contents of each of which are hereby incorporated as reference, each provide a method for forming a chalcogenide semiconductor film, which includes steps of coating a solution including an ink composition to form a layer on a substrate and heating the layer to form the chalcogenide semiconductor film. The methods of absorber film formation do not require vacuum equipments, and accordingly the manufacturing cost is reduced. Nevertheless, there is still a need for an improved ink composition for convenient and efficient film formation on substrates.